The lens–F0 distance measured by a beam in an isolated
corneal lens might differ somewhat from that in a living
specimen, since in vivo the medium posterior to the corneal
lens is an extracellular matrix of unknown refractive index,
whereas we used water. However, the distance may not differ
significantly between the two, since it has been reportedin many insects that the refractive index for vitreous bodies
ranges from 1.3 to 1.4 (e.g., Meyer-Rochow, 1974). In the
present study the beam converged at a distance of 50–
67 lm (58 ± 5 lm) from the corneal lens, whereas in
semi-thin sections the retina occurs 47–68 lm (55 ± 6 lm)
beneath the posterior margin of the corneal lens. These
data suggest that the posterior focal plane lay just on the
surface of the rhabdom layer in the central part of the retina.
This conclusion may be supported by data obtained in
right and left eyes for individual larvae as shown in Table
3, in which the differences between lens–retina distance and
lens–F0 distance are less than 10 lm in 8 out of 11 larvae.